Generally, a magnetic tape is widely used as the tape to record and keep information. A problem to make high density of the magnetic tape exists in how to increase recording density of information per unit area. Therefore, for example, although making a width of a single data track in which data is written as small as possible in a tape width direction, thereby mag a number of data tracks more (improvement of line density), and a substantial increase of a recording capacity is intended, there exists a limit only relying on machine accuracy of running device of a magnetic tape because it becomes difficult for a reproduce head to completely trace the data tracks when reproducing recorded signals in case of making the width of the data tracks small in the tape width direction.
On the other hand, in a recent high-density magnetic tape, a servo control mechanism has become to been built in, adding to a similar improvement of running device of a conventional magnetic tape. The servo control mechanism is designed to be able to completely trace data tracks on a magnetic tape by controlling a record head and reproduce head for a running magnetic tape.
In order to utilize servo control, firstly write servo signals on a magnetic tape using it in advance. Then, control a write head and reproduce head of a magnetic tape drive in positions of data tracks recording data with standardizing servo signals. Thus, high density by making a width of a data track in a tape width direction becomes possible.
Although there exist various formats as the servo signals, a system called a timing based servo recognizing a magnetic head position at a timing when pulses appear in read servo signals is known.
In a magnetic tape system using such the timing based servo system, it is important to accurately write servo signals on a magnetic tape. That is, the timing based servo assumes that the servo signals are accurately written, so unless they are written at an accurate timing (interval), there exist a possibility that track positions are mistaken with a magnetic tape drive.
Therefore, in order to write the servo signals at an accurate timing, with running the magnetic tape at a constant accurate speed, a exclusive device called a servo writer writing the servo signals with a fixed servo signal write head (hereinafter simply referred to as “write head”) is used.
Here, in order to run a magnetic tape, constant tension has to be given to the magnetic tape. Therefore, a servo writer keeps the magnetic tape tension constant feedback-controlling by a tension adjusting means, however operation for tension adjustment of the tension abusing means gives the magnetic tape small vibration, thereby accurately the servo signals being impeded.
As a prior technical document described about such the tension adjusting means, for example, see FIGS. 4, 5, and the like in Japan paten laid open publication 2001-167493.
The tension adjustment described above is generally thought to be better to detect tension deviation as soon as possible and to control a capstan and the like so as to early modify the tension, however in the case of a magnetic tape the tape itself is an elastic body and expands, thereby the tape being unable to follow if early controlled over a certain degree. Accordingly, in a current tape, it is near a limit to accurately write the servo signals by early performing feedback control.
In addition, although the magnetic tape vibration is due to the feedback control, an inconveniency occurs in some cases on the contrary if small deviation in the servo signals written on the magnetic tape occurs by early performing the feedback control of the tape tension. That is, in the magnetic tape drive, because in the case that data is recorded with a magnetic head being controlled as instructed by erroneously written servo signals, the magnetic head operates also in reproducing data same as in recording data, notwithstanding eventually data tracks being formed bent, there exists no problem thanks to the data tracks being able to be traced in reproducing. However, when the servo signals are written on the magnetic tape with a drastic timing error (change) which the magnetic head cannot follow, the magnetic head cannot always operate same in reproducing data and recording data and it causes an off-track (derailing of the reproduce head from a data track)